<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>canvas 3圆</title>
</head>
<body>
	<canvas id="app" width="500" height="600"></canvas>
	<script type="text/javascript">
		
		function draw0(id){
			var canvas = document.getElementById(id);
			if (canvas == null) {
				return false;
			}

			var context = canvas.getContext('2d');
			context.beginPath();
			context.arc(200, 150, 100, 0, Math.PI * 2, true);
			//不关闭路径路径会一直保留下去，当然也可以利用这个特点做出意想不到的效果
			context.closePath();
			context.fillStyle = 'rgba(0,255,0,0.25)';
			context.fill();
		}
		draw0("app")
	</script>
</body>
</html>